FIG. 1 is a schematic block diagram of a communication system comprising a host terminal 2 and an external wireless cellular modem 4, the modem 4 comprising an antenna 5. The host terminal 2 may be a user terminal, and may for example take the form of a desktop computer, laptop computer, tablet style computer, or mobile phone (which may be referred to as a “smart phone”). The modem 4 may, for example, take the form of a dongle for plugging into the host terminal 2. Alternatively the modem 4 may take the form of a mobile phone handset which, as well as being a conventional mobile telephone, can be connected to the host terminal 2 so as to act as an external cellular modem for the host terminal 2. The modem 4 is external to the host terminal 2 in that it is a separate unit housed in a separate casing, but which is connected or connectable to the host terminal 2 by means of a wired or wireless connection.
The system also comprises a mobile cellular network 6 such as a 3GPP network. The mobile network 6 comprises one or more access points 12 operatively coupled to a plurality of antennas 10 and GGSN (Gateway GPRS Support Node) 7. For example the one or more access points 12 may take the form of one or more instances of a suitable access point protocol such as one or more APNs (access point names), which may be implemented on one or more physical nodes of the mobile network 6. The system further comprises another, packet-based network 8, such as a wide area internetwork such as the Internet, comprising a plurality of network areas 14 and routers or gateways 16.
For connecting to the 3GPP network 6 or other such cellular network, the modem 4 comprises first physical interface apparatus comprising a wireless transceiver, typically in the form of a radio frequency (RF) transceiver and an antenna 5. This interface apparatus of the modem 4 connects via an antenna 10 of the cellular network 6 to an access point (e.g. APN) 12 of the cellular network 6, which is operatively coupled to the GGSN 7 of the mobile cellular network 6. For example, if the mobile cellular network 6 is a 3GPP network, then the connection between the modem 4 and a 3GPP network may be called a PDP (Packet Data Protocol) context in 2G or 3G terminology, and an EPS (Evolved Packet System) bearer context in LTE (Long Term Evolution standards). The physical medium of the connection is typically a radio channel such as a 2G, 3G or LTE radio channel and the protocol that drives it may comprise a set of protocol layers as defined for example by 3GPP. Each of the one or more access points 12 (e.g. APNs) connects on to a respective router 16 of the Internet 8. The first router 16 encountered on the route onwards from the mobile cellular network 6, i.e. the router immediately connecting to the APN 12, is the gateway between the cellular network 6 and the Internet 8.
For connecting to the host terminal 2, the modem 4 comprises second physical interface apparatus. The second interface, between the host 2 and modem 4, could for example comprise a wired connection such as USB, or a short-range wireless transceiver such as an infrared connection or a radio frequency connection (e.g. Bluetooth).
The host terminal 2 is installed with one or more applications 18 which when executed on the host terminal 2 send and/or receive communications over the Internet 8 via the first and second interface apparatuses, wireless cellular network 6 and the relevant gateway 16, supported by the GGSN 7. The system allows the host terminal to access the Internet 8, e.g. using Internet Protocol version 6 (IPv6) and/or Internet Protocol version 4 (IPv4). The host terminal 2 may be installed with and run one or more applications 18i configured to make use of a combination of IPv4 and IPv6 features or that can operate in modes based on either IPv4 or IPv6, and/or one or more applications 18ii that operate exclusively based on use of IPv6. To this end, the modem 4 may be operable to establish a context 20i with an APN 12i which together can support either IPv4 or IPv6, and/or to establish a context 20ii with an APN 12ii which together operate exclusively based on IPv6.
The protocol stack which drives the physical connection between the host terminal 2 (via the modem 4) and the access point 12 (e.g. APN) will include a link layer protocol. Examples of link layer protocols include Ethernet and PPP (Point-to-Point Protocol). The link layer protocol is the protocol that operates at the link layer, i.e. which handles the transfer of data at a level between only adjacent nodes of a wide area network or within a local area network segment, i.e. between only terminals, routers or local network segments that are adjacent to one another (as opposed for example to Internet layer protocol which routes over multiple routing hops across multiple nodes, or transport layer protocol which is concerned with end-to-end communication issues). That is to say, the link layer protocol handles transmission at a level having visibility of only a single link or only a single hop, not multiple links or multiple hops (a link may comprise a local switch or bridge but may not extend beyond the next router, i.e. may not include more than one routing hop). A link layer protocol of interest is the protocol concerned with conveying data between a terminal such as host terminal 2 and an access point 12 of a mobile cellular network such as 3GPP network 6 (via modem 4), but not at a higher level concerned with finding a route over multiple hops between more than two of the nodes 2, 16.
For the purpose of link layer transmission, nodes such as routers and gateways 16 and terminals 2 may be allocated a link-layer identifier which may be referred to as a Link Local Identifier (or equivalently for the purpose of this application, an interface identifier, or lid). The link layer identifier may be or form a part of a link layer address, which may be referred to as link local address (LLA). For example in 3GPP and IETF standards, the source LLA identifying a terminal such as the host terminal 2 as the source of a particular communication may be built from the Link Local Identifier for the terminal 2 concatenated with a global prefix for the gateway 16 to which it is connecting or connected. The LLA or other such link-layer address provides a unique link address for a device connected to a packet-based network 8 such as an IPv6 network.
In 3GPP and IETF standards, it is specified that the IPv6 Dynamic Address Allocation shall be performed using a Link Local Identifier provided by the GGSN (Gateway GPRS Support Node). For example, 3GPP TS 24.008 par.6.1.2A.2 states that if a terminal requests allocation of an IPv6 address, the network constructs it of two parts: a “/64 IPv6 prefix” and an interface identifier [Link Local Identifier] of 64 bits in length; and that the interface identifier is only used for building a unique link local IPv6 address. 3GPP TS 23.60 par.9.2.1.1 states that to ensure the link-local address generated by the terminal does not collide with the link-local address of the GGSN, the GGSN shall provide an interface identifier [Link Local Identifier] to the terminal and the terminal shall use this interface identifier to configure its link local address (see also RFC 4862 [99]).
Generally speaking, in any arrangement where a terminal wishes to access the internet or other such packet-based network via a mobile cellular network, then an element of one of the networks may wish to allocate a link-layer identifier to the host terminal for use in setting up the access. For example, this could be so that the link-layer identifier does not coincide with that of another terminal connected or attempting to connect to the same gateway.